Tread for a tire having a rigidity gradient

ABSTRACT

A tire tread comprising a rubber composition based on (phr: parts by weight per hundred parts of elastomer):
         (i) a diene elastomer;   (ii) more than 50 phr of an inorganic filler as reinforcing filler;   (iii) between 2 and 15 phr of an (inorganic filler/diene elastomer) coupling agent;   (iv) between 1 and 10 phr of a methylene acceptor, and   (v) between 0.5 and 5 phr of a methylene donor.
 
This tread has, after mechanical running-in of the tire for which it is intended (“auto-accommodation”), a rigidity gradient which increases radially from the surface towards the inside of the tread. Use of such a tread for the manufacturing or recapping of tires. Tires comprising a tread according to the invention, in particular of the snow or ice type (“winter” tires).

The present application is a continuation of International ApplicationNo. PCT/EPO1/08566, filed 25 Jul. 2001, published in French with anEnglish Abstract on 7 Feb. 2001, under PCT Article 21(2), which claimspriority to French Patent Application No. FR00/10094, filed 31 Jul.2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to treads for tires and to rubbercompositions used for the manufacturing of such treads.

It relates more particularly to treads for tires having a low rollingresistance, reinforced majoritarily by reinforcing inorganic fillerssuch as siliceous or aluminous fillers, these treads being intended inparticular for tires fitted on vehicles such as motorcycles, passengercars, vans or heavy vehicles.

2. Description of the Related Art

Since fuel economies and the need to protect the environment have becomepriorities, it has proved necessary to produce tires having reducedrolling resistance. This has been made possible in particular due to thediscovery of new rubber compositions reinforced with specific inorganicfillers referred to as “reinforcing” fillers, which are capable ofrivalling conventional carbon black from the reinforcing point of view,and furthermore offering these compositions a low hysteresis, which issynonymous with lesser rolling resistance for the tires comprising them.

Such rubber compositions, comprising reinforcing inorganic fillers ofthe siliceous or aluminous type, have for example been described inpatents or patent applications EP-A-0 501 227 (or U.S. Pat. No.5,227,425), EP-A-0 735 088 (or U.S. Pat. No. 5,852,099), EP-A-0 810 258(or U.S. Pat. No. 5,900,449), EP-A-0 881 252, WO99/02590, WO99/02601,WO99/02602, WO99/28376, WO00/05300 and WO00/05301.

Mention will be made in particular of documents EP-A-0 501 227, EP-A-0735 088 or EP-A-0 881 252, which disclose diene rubber compositionsreinforced with precipitated silicas of the highly dispersible type,such compositions making it possible to manufacture treads having asignificantly improved rolling resistance, without adversely affectingthe other properties, in particular those of grip, endurance and wearresistance. Such compositions having such an excellent compromise ofcontradictory properties are also described in applications EP-A-0 810258 and WO99/28376, with specific aluminous fillers (alumina oraluminium oxide-hydroxides) of high dispersibility as reinforcinginorganic fillers.

Ideally, a tire tread must meet a large number of technical demands,which are frequently contradictory, including: high wear resistance, lowrolling resistance, very good grip both on dry ground and on wet,snow-covered or icy ground, while offering the tire a very good level ofroad behaviour (“handling”) on an automobile, in particular high driftthrust (“cornering”).

To improve the road behaviour, it is known that greater rigidity of thetread is desirable, this stiffening of the tread possibly being obtainedfor example by increasing the amount of reinforcing filler or byincorporating certain reinforcing resins into the rubber compositionsconstituting these treads.

However, such stiffening of the tread, at the very least for its surfacepart which is in contact with the ground during running of the tire, inknown manner impairs, most frequently in crippling manner, theproperties of grip on wet, snow-covered or icy ground.

To meet these two contradictory demands, namely road behaviour and grip,it has essentially been proposed hitherto to use composite treads (i.e.hybrid treads), formed by two radially superposed layers (“cap-basestructure”) of different rigidity, formed of two rubber compositions ofdifferent formulations: the radially outer layer, in contact with theroad, is formed of the most flexible composition, in order to meet thegrip requirements; the radially inner layer is formed of the most rigidcomposition, in order to meet the road-behaviour requirements.

Such a solution however has numerous disadvantages:

-   -   first of all, the manufacturing of a composite tread is by        definition more complex and therefore more costly than that of a        conventional tread, and requires in particular the use of        complex coextrusion machines;    -   during manufacturing, after cutting out the tread to the correct        dimensions once it has emerged from the extruder, it is        furthermore necessary to manage discarding of material of        different natures, which further substantially increases the        production costs;    -   finally, and this is not the least of the disadvantages, once        the radially outer (flexible) part of the tread has become worn,        it is the initially inner part of the tread which comes into        contact with the road: then, of course, one has the        disadvantages of an excessively rigid tread, with unsatisfactory        performance from the point of view of the technical compromise        initially intended.

BRIEF SUMMARY OF THE INVENTION

Now, the Applicants have discovered during their research that aspecific rubber composition, based on a high amount of reinforcinginorganic filler and a methylene acceptor/donor system, makes itpossible, owing to an unexpected “auto-accommodation” phenomenon, toobtain a tread having a true rigidity gradient, radially increasing fromthe surface towards the inside of the tread. This rigidity gradient isnot only obtained simply and economically, but also durably, thus makingit possible to keep the compromise between grip and road behaviour ofthe tires at a very high level, throughout the life of the latter.

Consequently, a first subject of the invention relates to a tire treadformed, at least in part, of a rubber composition based on at least(phr: parts by weight per hundred parts of elastomer):

(i) a diene elastomer;

(ii) more than 50 phr of an inorganic filler as reinforcing filler;

(iii) between 2 and 15 phr of an (inorganic filler/diene elastomer)coupling agent;

(iv) between 1 and 10 phr of a methylene acceptor, and

(v) between 0.5 and 5 phr of a methylene donor.

The subject of the invention is also the use of such a tread for themanufacturing of new tires or the recapping of worn tires.

The tread according to the invention is particularly suited to tiresintended to be fitted on passenger vehicles, 4×4 vehicles (having 4driving wheels), motorcycles, vans and heavy vehicles.

The subject of the invention is also these tires themselves, when theycomprise a tread according to the invention. It relates in particular totires of “winter” type intended for snow-covered or icy roads.

Another subject of the invention is a process for preparing asulphur-vulcanisable tire tread, liable to have, after mechanicalrunning-in of the tire for which it is intended, a rigidity gradientwhich increases radially from the surface towards the inside of thetread, this process being characterised in that it comprises thefollowing steps:

-   -   incorporating in a diene elastomer, in a mixer:        -   more than 50 phr of an inorganic filler as reinforcing            filler;        -   between 2 and 15 phr of an (inorganic filler/diene            elastomer) coupling agent;        -   between 1 and 10 phr of a methylene acceptor,    -   by thermomechanically kneading the entire mixture, in one or        more stages, until a maximum temperature of between 130° C. and        200° C. is reached;    -   cooling the entire mixture to a temperature of less than 100°        C.;    -   then incorporating:        -   between 0.5 and 5 phr of a methylene donor,        -   a vulcanisation system;    -   kneading the entire mixture until a maximum temperature less        than 120° C. is reached;    -   extruding or calendering the rubber composition thus obtained,        in the form of a tire tread.

The invention and its advantages will be readily understood in the lightof the description and examples of embodiment which follow, and of FIG.1 relating to these examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship of the variation of rigidity(modulus M10) as a function of the depth (“e” in mm) in differenttreads.

DETAILED DESCRIPTION OF THE INVENTION I. Measurements and Tests Used

The treads and rubber compositions constituting these treads arecharacterised, before and after curing, as indicated hereafter.

I-1. Mooney plasticity. An oscillating consistometer such as describedin French Standard NF T 43-005 (November 1980) is used. The Mooneyplasticity is measured in accordance with the following principle: theraw composition (i.e. before curing) is moulded in a cylindricalenclosure heated to 100° C. After one minute's preheating, the rotorturns within the test piece at 2 rpm, and the torque used formaintaining this movement is measured after four minutes' rotation. TheMooney plasticity (ML 1+4) is expressed in “Mooney units” (MU, with 1MU=0.83 Newton.meter).

I-2. Scorching time. The measurements are effected at 130° C., inaccordance with French Standard NF T 43-005. The evolution of theconsistometric index as a function of time makes it possible todetermine the scorching time for the rubber compositions, assessed inaccordance with the above standard by the parameter T5 (case of a largerotor), expressed in minutes, and defined as being the time necessary toobtain an increase in the consistometric index (expressed in MU) of 5units above the minimum value measured for this index.

I-3. Tensile tests. These tensile tests make it possible to determinethe elasticity stresses and the properties at break. Unless indicatedotherwise, they are effected in accordance with French Standard NF T46-002 of September 1988. The nominal secant moduli (or apparentstresses, in MPa) at 10% elongation (M10), 100% elongation (M100) and300% elongation (M300) are measured in a second elongation (i.e. after acycle of accommodation to the amount of extension provided for themeasurement itself).

The nominal secant modulus is also measured at 10% elongation, after anaccommodation of 15% (i.e. an extension to 15% followed by relaxation to0%) and not 10% as above for the modulus M10. This so-called“accommodated” modulus is referred to as M10_(Ac). The breaking stresses(in MPa) and the elongations at break (in %) are also measured. Allthese tensile measurements are effected under normal conditions oftemperature (23±2° C.) and humidity (50±5% relative humidity), inaccordance with French standard NFT 40-101 (December 1979).

I-4. Shore A hardness. The Shore A hardness of the compositions aftercuring is assessed in accordance with ASTM Standard D 2240-86.

I-5. Dynamic properties. The dynamic properties ΔG* and tan(β)_(max) aremeasured on a viscoanalyser (Metravib VA4000), in accordance with ASTMStandard D5992-96. The response of a sample of vulcanised composition(cylindrical test piece of a thickness of 4 mm and a section of 400mm²), subjected to an alternating single sinusoidal shearing stress, ata frequency of 10 Hz, under normal conditions of temperature (23° C.) inaccordance with Standard ASTM D 1349-99, is recorded. Scanning waseffected at an amplitude of deformation of 0.1 to 50% (outward cycle),then of 50% to 1% (return cycle). The results used are the complexdynamic shear modulus (G*) and the loss factor tan(δ). For the returncycle, the maximum value of tan(δ) which is observed is indicated(tan(δ)_(max)), and the deviation in the complex modulus (ΔG*) betweenthe values at 0.15% and 50% deformation (Payne effect).

I-6. Tests on tires. A) Shore A hardness: It is measured on the outersurface of the tread, the one in contact with the ground, in accordancewith the aforementioned standard ASTM D 2240-86.

B) Drift thrust: Each tire tested is mounted on a wheel of suitabledimension and inflated to 2.2 bar. It is made to run at a constant speedof 80 km/h on a suitable automatic machine (“flat-belt” type testmachine sold by MTS). The load “Z” is varied, at a drift angle of 1degree, and the rigidity or drift thrust “D” (corrected for the thrustat zero drift) is measured in known manner, by recording the transverseforce on the wheel as a function of this load Z using sensors. The driftthrust indicated in the tables is the gradient at the origin of thecurve D(Z).

C) Accommodation or mechanical running-in: Some of the characteristicsof the tires tested may be measured both on new tires (that is to say inthe initial state, before any running) and on tires which have undergonemechanical “accommodation” of their treads.

“Mechanical accommodation” is to be understood to mean here simplerunning-in of the tire by means of which its tread is placed in contactwith the ground during running, that is to say in working conditions,for several tens of seconds or several minutes at the most. Thisrunning-in operation may be carried out on an automatic running machineor directly on an automobile, and effected in various ways, for exampleby simply running in a straight line of several tens or hundreds ofmeters, by longitudinal braking or alternatively by drifting of the tire(bends), the important thing being to start making the tread “work”under normal conditions of use.

For the requirements of the tire tests which follow, the mechanicalaccommodation is achieved, unless indicated otherwise in the rest of thetext, by what is called “standard” running-in consisting of simplerunning in a straight line over a length of 400 meters at a speed of 60km/h, on a given automobile, without drifting or cambering imposed onthe tire, followed by moderate longitudinal braking (braking distancefrom 30 to 40 meters) to stop the vehicle. This “standard running-in” isfurthermore effected under normal conditions of pressure (thoserecommended by the manufacturer of the vehicle used) and load (1 persononly on board the vehicle).

D) Braking on damp ground: The tires are mounted on an automobile fittedwith an ABS braking system and the distance necessary to go from 40 km/hto 10 km/h upon sudden braking on wetted ground (asphalt concrete) ismeasured. A value greater than that of the control, arbitrarily set to100, indicates an improved result, that is to say a shorter brakingdistance.

E) Grip on damp ground: To assess the grip performance on damp ground,the behaviour of the tires mounted on a given automobile travellinground a circuit comprising numerous bends and wetted so as to keep theground damp, under limit speed conditions is analysed.

On one hand the minimum time necessary to cover the entire circuit ismeasured; a value less than that of the control, arbitrarily set to 100,indicates an improved result, that is to say a shorter travelling time.

The professional driver of the vehicle, on the other hand, assigns asubjective overall mark for road behaviour of the vehicle—and thereforeof the tires—on this wetted circuit comprising bends; a mark greaterthan that of the control, arbitrarily set to 100, indicates improvedoverall behaviour.

II. Conditions of Carrying Out the Invention

The treads according to the invention are formed, at least in part, of arubber composition based on at least: (i) a (at least one) dieneelastomer; (ii) a minimum quantity (more than 50 phr) of a (at leastone) inorganic filler as reinforcing filler; (iii) a (at least one)coupling agent (between 2 and 15 phr) providing the bond between thereinforcing inorganic filler and this diene elastomer; (iv) a (at leastone) acceptor (between 1 and 10 phr) and (v) a (at least one) methylenedonor (between 0.5 and 5 phr).

Of course, the expression composition “based on” is to be understood tomean a composition comprising the mix and/or the product of reaction insitu of the various constituents used, some of these base constituents(for example, the coupling agent, the methylene acceptor and donor)being liable to, or intended to, react together, at least in part,during the different phases of manufacturing of the treads, inparticular during the vulcanisation (curing) thereof.

II-1. Diene elastomer. “Diene” elastomer or rubber is understood tomean, generally, an elastomer resulting at least in part (i.e. ahomopolymer or a copolymer) from diene monomers (monomers bearing twodouble carbon—carbon bonds, whether conjugated or not). “Essentiallyunsaturated” diene elastomer is understood here to mean a dieneelastomer resulting at least in part from conjugated diene monomers,having a content of members or units of diene origin (conjugated dienes)which is greater than 15% (mol %). Thus, for example, diene elastomerssuch as butyl rubbers or copolymers of dienes and of alpha-olefins ofthe EPDM type do not fall within this definition, and may on thecontrary be described as “essentially saturated” diene elastomers (lowor very low content of units of diene origin which is always less than15%). Within the category of “essentially unsaturated” diene elastomers,“highly unsaturated” diene elastomer is understood to mean in particulara diene elastomer having a content of units of diene origin (conjugateddienes) which is greater than 50%.

These general definitions being given, the person skilled in the art oftires will understand that the present invention is used first andforemost with highly unsaturated diene elastomers, in particular with:

(a)—any homopolymer obtained by polymerisation of a conjugated dienemonomer having 4 to 12 carbon atoms;

(b)—any copolymer obtained by copolymerization of one or more conjugateddienes with each other or with one or more vinyl-aromatic compoundshaving 8 to 20 carbon atoms.

Suitable conjugated dienes are, in particular, 1,3-butadiene,2-methyl-1,3-butadiene, 2,3-di(C₁–C₅ alkyl)-1,3-butadienes such as, forinstance, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene,2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, anaryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene. Suitablevinyl-aromatic compounds are, for example, styrene, ortho-, meta- andpara-methylstyrene, the commercial mixture “vinyltoluene”,para-tert.-butylstyrene, methoxystyrenes, chlorostyrenes,vinylmesitylene, divinylbenzene and vinylnaphthalene.

The copolymers may contain between 99% and 20% by weight of diene unitsand between 1% and 80% by weight of vinyl-aromatic units. The elastomersmay have any microstructure, which is a function of the polymerisationconditions used, in particular of the presence or absence of a modifyingand/or randomising agent and the quantities of modifying and/orrandomising agent used. The elastomers may for example be block,statistical, sequential or microsequential elastomers, and may beprepared in dispersion or in solution; they may be coupled and/orstarred or alternatively functionalised with a coupling and/or starringor functionalising agent.

Preferred are polybutadienes, and in particular those having a contentof 1,2-units between 4% and 80%, or those having a content of cis-1,4[bonds] of more than 80%, polyisoprenes, butadiene-styrene copolymers,and in particular those having a styrene content of between 5% and 50%by weight and, more particularly, between 20% and 40%, a content of1,2-bonds of the butadiene fraction of between 4% and 65%, and a contentof trans-1,4 bonds of between 20% and 80%, butadiene-isoprene copolymersand in particular those having an isoprene content of between 5% and 90%by weight and a glass transition temperature (“Tg”—measured inaccordance with ASTM Standard D3418-82) of between −40° C. and −80° C.,isoprene-styrene copolymers and in particular those having a styrenecontent of between 5% and 50% by weight and a Tg of between −25° C. and−50° C. In the case of butadiene-styrene-isoprene copolymers, thosewhich are suitable are in particular those having a styrene content ofbetween 5% and 50% by weight and, more particularly, between 10% and40%, an isoprene content of between 15% and 60% by weight, and moreparticularly between 20% and 50%, a butadiene content of between 5% and50% by weight, and more particularly between 20% and 40%, a content of1,2-units of the butadiene fraction of between 4% and 85%, a content oftrans-1,4 units of the butadiene fraction of between 6% and 80%, acontent of 1,2- plus 3,4-units of the isoprene fraction of between 5%and 70%, and a content of trans-1,4 units of the isoprene fraction ofbetween 10% and 50%, and more generally any butadiene-styrene-isoprenecopolymer having a Tg of between −20° C. and −70° C.

In summary, particularly preferably, the diene elastomer of thecomposition according to the invention is selected from the group ofhighly unsaturated diene elastomers which consists of polybutadienes(BR), synthetic polyisoprenes (IR), natural rubber (NR), butadienecopolymers, isoprene copolymers and mixtures of these elastomers.

Such copolymers are more preferably selected from the group whichconsists of butadiene-styrene copolymers (SBR), butadiene-isoprenecopolymers (BIR), isoprene-styrene copolymers (SIR),isoprene-butadiene-styrene copolymers (SBIR) and mixtures of suchcopolymers.

The tread according to the invention is preferably intended for apassenger-car tire. In such a case, the diene elastomer is preferably anSBR copolymer, in particular an SBR prepared in solution, preferablyused in a mixture with a polybutadiene; more preferably, the SBR has acontent of styrene of between 20% and 30% by weight, a content of vinylbonds of the butadiene fraction of between 15% and 65%, a content oftrans-1,4 bonds of between 15% and 75% and a Tg of between −20° C. and−55° C., and the polybutadiene has more than 90% cis-1,4 bonds.

The compositions of the treads of the invention may contain a singlediene elastomer or a mixture of several diene elastomers, the dieneelastomer(s) possibly being used in association with any type ofsynthetic elastomer other than a diene one, or even with polymers otherthan elastomers, for example thermoplastic polymers.

II-2. Reinforcing inorganic filler. “Reinforcing inorganic filler” is tobe understood here to mean any inorganic or mineral filler, whatever itscolour and its origin (natural or synthetic), also referred to as“white” filler or sometimes “clear” filler in contrast to carbon black,this inorganic filler being capable, on its own, without any other meansthan an intermediate coupling agent, of reinforcing a rubber compositionintended for the manufacturing of a tire tread, in other words which iscapable of replacing a conventional tire-grade carbon black (for treads)in its reinforcement function.

Preferably, the reinforcing inorganic filler is a filler of thesiliceous or aluminous type, or a mixture of these two types of fillers.

The silica (SiO₂) used may be any reinforcing silica known to the personskilled in the art, in particular any precipitated or pyrogenic silicahaving a BET surface area and a specific CTAB surface area both of whichare less than 450 m²/g, preferably from 30 to 400 m²/g. Highlydispersible precipitated silicas (referred to as “HD”) are preferred, inparticular when the invention is used for the manufacturing of tireshaving a low rolling resistance; “highly dispersible silica” isunderstood in known manner to mean any silica having a substantialability to disagglomerate and to disperse in an elastomeric matrix,which can be observed in known manner by electron or optical microscopyon thin sections. As non-limitative examples of such preferred highlydispersible silicas, mention may be made of the silicas BV3380 andUltrasil 7000 from Degussa, the silicas Zeosil 1165 MP and 1115 MP fromRhodia, the silica Hi-Sil 2000 from PPG, the silicas Zeopol 8715 or 8745from Huber, and treated precipitated silicas such as, for example, thealuminium-“doped” silicas described in application EP-A-0 735 088.

The reinforcing alumina (Al₂O₃) preferably used is a highly dispersiblealumina having a BET surface area from 30 to 400 m²/g, more preferablybetween 60 and 250 m²/g, an average particle size at most equal to 500nm, more preferably at most equal to 200 nm, as described in theaforementioned application EP-A-0 810 258. Non-limitative examples ofsuch reinforcing aluminas are in particular the aluminas A125 or CR125(from Baïkowski), APA-100RDX (from Condéa), Aluminoxid C (from Degussa)or AKP-G015 (Sumitomo Chemicals). The invention can also be implementedby using as reinforcing inorganic filler the specific aluminium(oxide-)hydroxides such as described in the aforementioned applicationWO99/28376.

The physical state in which the reinforcing inorganic filler is presentis immaterial, whether it be in the form of a powder, microbeads,granules, balls or any other densified form.

Of course, “reinforcing inorganic filler” is also understood to meanmixtures of different reinforcing inorganic fillers, in particular ofhighly dispersible siliceous and/or aluminous fillers such as describedabove.

When the treads of the invention are intended for tires of low rollingresistance, the reinforcing inorganic filler used, in particular if itis silica, preferably has a BET surface area of between 60 and 250 m²/g,more preferably between 80 and 230 m²/g.

The inorganic filler used as reinforcing filler must be present in anamount greater than 50 phr, which is one of the essentialcharacteristics of the invention. This reinforcing inorganic filler mayconstitute all or the majority of the total reinforcing filler, in thislatter case associated for example with a minority quantity of carbonblack.

Preferably, the amount of reinforcing inorganic filler is between 60 and120 phr, more preferably still within a range from 70 to 100 phrapproximately, in particular when the tread is intended for apassenger-car tire. The person skilled in the art will readilyunderstand that the optimum will be different according to the nature ofthe reinforcing inorganic filler used and according to the type of tirein question, for example a tire for a motorcycle, passenger vehicle oralternatively for a utility vehicle such as a van or a heavy vehicle.

Preferably, in the tread according to the invention, the reinforcinginorganic filler constitutes more than 80% by weight of the totalreinforcing filler, more preferably more than 90% by weight (or evenall) of this total reinforcing filler.

However, without significantly affecting the technical effect desired, asmall quantity of carbon black, preferably less than 20%, morepreferably still less than 10% by weight relative to the quantity oftotal reinforcing filler, may be used.

The carbon black, if it is used, is preferably present in an amount ofbetween 2 and 15 phr, more preferably between 4 and 12 phr. It can beused in particular as a simple black pigmentation agent, oralternatively to protect the tread from different sources of atmosphericageing such as ozone, oxidation or UV radiation. On the other hand, itis known that certain rubber-making additives, in particular certaincoupling agents, are available in a form supported by carbon black, theuse of such additives therefore involving the incorporation, in a smallproportion, of carbon black. Suitable carbon blacks are any carbonblacks, in particular the blacks of the type HAF, ISAF and SAF, whichare conventionally used in tires, and particularly in treads for thesetires; as non-limitative examples of such blacks, mention may be made ofthe blacks N115, N134, N234, N339, N347 and N375.

In the present specification, the “BET” specific surface area isdetermined in known manner, in accordance with the method of Brunauer,Emmett and Teller described in “The Journal of the American ChemicalSociety”, vol. 60, page 309, February 1938, and corresponding to FrenchStandard NFT 45-007 (November 1987); the CTAB specific surface area isthe external surface area determined in accordance with the sameStandard NF T 45-007.

Finally, as filler equivalent to such a reinforcing inorganic filler,there could be used a reinforcing filler of organic type, in particulara carbon black, covered at least in part with an inorganic layer (forexample, a layer of silica), which for its part requires the use of acoupling agent to provide the connection to the elastomer.

II-3. Coupling agent. In known manner, in the presence of a reinforcinginorganic filler, it is necessary to use a coupling agent, also referredto as bonding agent, the function of which is to provide the connectionor bond between the surface of the particles of this inorganic fillerand the diene elastomer, while facilitating the dispersion of thisinorganic filler within the elastomeric matrix during thethermomechanical kneading.

It will be recalled that (inorganic filler/elastomer) “coupling agent”is to be understood to mean an agent capable of establishing asufficient chemical and/or physical connection between the inorganicfiller and the elastomer; such a coupling agent, which is consequentlyat least bifunctional, has, for example, the simplified general formula“Y-T-X”, in which:

-   -   Y represents a functional group (function “Y”) which is capable        of bonding physically and/or chemically with the inorganic        filler, such a bond being able to be established, for example,        between a silicon atom of the coupling agent and the surface        hydroxyl (OH) groups of the inorganic filler (for example,        surface silanols in the case of silica);    -   X represents a functional group (function “X”) which is capable        of bonding physically and/or chemically with the diene        elastomer, for example by means of a sulphur atom;    -   T represents a divalent organic group making it possible to link        Y and X.

The coupling agents must particularly not be confused with simple agentsfor covering the inorganic filler which, in known manner, may comprisethe function Y which is active with respect to the inorganic filler butare devoid of the function X which is active with respect to theelastomer.

(Silica/diene elastomer) coupling agents, of variable effectiveness,have been described in a very large number of documents and arewell-known to the person skilled in the art. Any known coupling agentlikely to ensure, in the diene rubber compositions which can be used forthe manufacturing of tire treads, the bonding between a reinforcinginorganic filler such as silica and a diene elastomer, in particularorganosilanes or polyfunctional polyorganosiloxanes bearing thefunctions X and Y mentioned above, may be used.

In particular polysulphurized silanes, which are referred to as“symmetrical” or “asymmetrical” depending on their specific structure,are used, such as those described for example in the patents or patentapplications FR 2 149 339, FR 2 206 330, U.S. Pat. No. 3,842,111, U.S.Pat. No. 3,873,489, U.S. Pat. No. 3,978,103, U.S. Pat. No. 3,997,581,U.S. Pat. No. 4,002,594, U.S. Pat. No. 4,072,701, U.S. Pat. No.4,129,585, U.S. Pat. No. 5,580,919, U.S. Pat. No. 5,583,245, U.S. Pat.No. 5,650,457, U.S. Pat. No. 5,663,358, U.S. Pat. No. 5,663,395, U.S.Pat. No. 5,663,396, U.S. Pat. No. 5,674,932, U.S. Pat. No. 5,675,014,U.S. Pat. No. 5,684,171, U.S. Pat. No. 5,684,172, U.S. Pat. No.5,696,197, U.S. Pat. No. 5,708,053, U.S. Pat. No. 5,892,085, EP 1 043357.

Particularly suitable for implementing the invention, without thedefinition below being limitative, are so-called “symmetrical”polysulphurized silanes which satisfy the following general formula (I):Z-A-S_(n)-A-Z, in which:  (I)

-   -   n is an integer from 2 to 8 (preferably from 2 to 5);    -   A is a divalent hydrocarbon radical (preferably C₁–C₁₈ alkylene        groups or C₆–C₁₂ arylene groups, more particularly C₁–C₁₀        alkylenes, notably C₁–C₄ alkylenes, in particular propylene);    -   Z corresponds to one of the formulae below:

in which:

-   -   the radicals R¹, which may or may not be substituted, and may be        identical or different, represent a C₁–C₁₈ alkyl group, a C₅–C₁₈        cycloalkyl group or a C₆–C₁₈ aryl group, (preferably C₁–C₆ alkyl        groups, cyclohexyl or phenyl, in particular C₁–C₄ alkyl groups,        more particularly methyl and/or ethyl).    -   the radicals R², which may or may not be substituted, and may be        identical or different, represent a C₁–C₁₈ alkoxyl group or a        C₅–C₁₈ cycloalkoxyl group (preferably a group selected from        among C₁–C₈ alkoxyls and C₅–C₈ cycloalkoxyls, more preferably        still a group selected from among C₁–C₄ alkoxyls, in particular        methoxyl and/or ethoxyl).

In the case of a mixture of polysulphurized alkoxysilanes in accordancewith Formula (I) above, in particular conventional, commerciallyavailable, mixes, the average value of the “n”s is a fractional number,preferably between 2 and 5, more preferably close to 4. However, theinvention may also be implemented advantageously for example withdisulphurised alkoxysilanes (n=2).

As examples of polysulphurized silanes, mention will be made moreparticularly of the polysulphides (in particular disulphides,trisulphides or tetrasulphides) ofbis-((C₁–C₄)alkoxyl-C₁–C₄)alkylsilyl(C₁–C₄)alkyl), such as for examplethe polysulphides of bis(3-trimethoxysilylpropyl) or ofbis(3-triethoxysilylpropyl). Of these compounds, in particularbis(3-triethoxysilylpropyl)tetrasulphide, abbreviated TESPT, of theformula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, or bis(triethoxysilylpropyl)disulphide,abbreviated TESPD, of the formula [(C₂H₅O)₃Si(CH₂)₃S]₂, are used.

TESPD is sold, for example, by Degussa under the name Si75 (in the formof a mixture of disulphide—75% by weight—and of polysulphides), oralternatively by Witco under the name Silquest A1589. TESPT is sold, forexample, by Degussa under the name Si69 (or X50S when it is supported to50% by weight on carbon black), or alternatively by Osi Specialtiesunder the name Silquest A1289 (in both cases, a commercial mixture ofpolysulphides having an average value of n which is close to 4).

By way of examples of coupling agents other than the aforementionedpolysulphurized alkoxysilanes, mention will be made in particular of thebifunctional polyorganosiloxanes such as described for example in theaforementioned application WO99/02602.

In the treads according to the invention, the content of coupling agentis preferably between 4 and 12 phr, more preferably between 3 and 8 phr.However, it is generally desirable to use as little as possible thereof.

Relative to the weight of reinforcing inorganic filler, the amount ofcoupling agent preferably represents between 0.5 and 15% by weightrelative to the quantity of reinforcing inorganic filler. Morepreferably, in the case of treads for tires for passenger vehicles, thecoupling agent is present in an amount less than 12%, or even less than10% by weight relative to this quantity of reinforcing inorganic filler.

The polysulphurized alkoxysilane could first be grafted (via thefunction “X”) on to the diene elastomer of the composition of theinvention, the elastomer thus functionalised or “precoupled” thencomprising the free function “Y” for the reinforcing inorganic filler.The polysulphurized alkoxysilane could also be grafted beforehand (viathe function “Y”) on the reinforcing inorganic filler, the thus“precoupled” filler then being able to be bonded to the diene elastomerby means of the free function “X”. However, it is preferred, inparticular for reasons of better processing of the compositions in theuncured state, to use the coupling agent, either grafted onto thereinforcing inorganic filler, or in the free (i.e. non-grafted) state.

There may possibly be associated with the coupling agent an appropriate“coupling activator”, that is to say, a body (single compound orassociation of compounds) which, when mixed with this coupling agent,increases the effectiveness of the latter. Coupling activators forpolysulphurized alkoxysilanes have for example been described in theaforementioned international applications WO00/05300 and WO00/05301,consisting of the association of a substituted guanidine, in particularN,N′-diphenylguanidine (abbreviated to “DPG”), with an enamine or a zincdithiophosphate. The presence of these coupling activators will make itpossible, for example, to keep the amount of coupling agent at apreferred level of less than 10%, or even less than 8% by weightrelative to the quantity of reinforcing inorganic filler, oralternatively to reduce the amount of reinforcing inorganic filler owingto the improved coupling with the diene elastomer.

II-4. Methylene Acceptor/Donor System (so-called “M.A.D.” system). Theterms “methylene acceptor” and “methylene donor” are well-known to theperson skilled in the art and are widely used to designate compoundsliable to react together to generate a three-dimensional reinforcingresin by condensation.

The rubber compositions of the treads of the invention contain, incombination, at least one methylene acceptor associated with at leastone methylene donor, which are intended to form in situ, after curing(vulcanisation) of the tread, a three-dimensional resin lattice which issuperposed and interpenetrates with the (inorganic filler/elastomer)lattice on one hand, and with the (elastomer/sulphur) lattice on theother hand (if the cross-linking agent is sulphur).

Methylene acceptors, in particular novolac resins, whether or notassociated with a methylene donor, had admittedly already been describedin rubber compositions, in particular those intended for tires or treadsfor tires, for applications as varied as grip or reinforcement:reference will be made, for example, to documents EP-A-0 967 244, U.S.Pat. No. 6,028,137, U.S. Pat. No. 6,015,851, U.S. Pat. No. 5,990,210,U.S. Pat. No. 5,872,167, U.S. Pat. No. 5,859,115 or EP-A-0 736 399, U.S.Pat. No. 5,840,113 or EP-A-0 875 532, U.S. Pat. No. 5,405,897, U.S. Pat.No. 5,049,418 and U.S. Pat. No. 4,837,266.

However, as far as the Applicant is aware, no document of the prior artdescribes the use in a tire tread, in the proportions set forth here, ofsuch an M.A.D. system in combination with a so high amount (more than 50phr, preferably more than 60 phr) of a reinforcing inorganic filler suchas silica.

A) Methylene acceptor. In known manner, the term “methylene acceptor”designates the reactant with which the methylene donor compound reactsby formation of methylene bridges (—CH2-), upon the curing of thecomposition, thus resulting in the formation in situ of thethree-dimensional resin lattice.

The methylene acceptor must be able to disperse perfectly in the rubbermatrix, at the same time as the reinforcing inorganic filler and itscoupling agent.

Particularly suitable are phenols, the generic name for hydroxylatedderivatives of arenes and the equivalent compounds; such a definitioncovers in particular monophenols, for example phenyl proper orhydroxybenzene, bisphenols, polyphenols (polyhydroxyarenes), substitutedphenols such as alkylphenols or aralkylphenols, for example bisphenols,diphenylolpropane, diphenylolmethane, naphthols, cresol, t-butylphenol,octylphenol, nonylphenol, xylenol, resorcinol or analogous products.

Preferably phenolic resins referred to as “novolac resins”, also calledphenol-aldehyde precondensates, resulting from the precondensation ofphenolic compounds and aldehydes, in particular formaldehyde, are used.In known manner, these novolac resins (also referred to as “two-stepresins”) are thermoplastic and require the use of a curing agent(methylene donor) to be cross-linked, unlike, for example, resols whichare thermohardening; they have sufficient plasticity not to interferewith the processing of the rubber composition. After cross-linking bythe methylene donor (they may then be referred to as “thermohardening”novolac resins), they are characterised in particular by a tighterthree-dimensional lattice than that of the resols.

The quantity of methylene acceptor must be between 1 and 10 phr; belowthe minimum indicated, the technical effect desired is inadequate,whereas beyond the maximum indicated there are the risks of excessivestiffening and excessive compromising of the hysteresis. For all thesereasons, a quantity of between 2 and 8 phr is more preferably selected,amounts lying within a range from 3 to 6 phr being particularlyadvantageous in the case of treads for passenger-car tires.

Finally, the quantity of methylene acceptor, in the ranges indicatedabove, is advantageously adjusted so as to represent between 2% and 15%,more preferably between 4% and 10%, by weight relative to the quantityof reinforcing inorganic filler.

B) Methylene donor. A curing agent, capable of cross-linking orhardening the methylene acceptor previously described, also commonlyreferred to as “methylene donor”, must be associated with this acceptor.The cross-linking of the resin is then caused during the curing of therubber matrix, by formation of (—CH₂—) bridges.

Preferably, the methylene donor is selected from the group consisting ofhexamethylenetetramine (abbreviated to HMT), hexamethoxymethylmelamine(abbreviated to HMMM or H3M), hexaethoxymethylmelamine, formaldehydepolymers such as p-formaldehyde, N-methylol melamine derivatives, ormixtures of these compounds. More preferably, a methylene donor selectedfrom among HMT, H3M or a mixture of these compounds is used.

The quantity of methylene donor must be between 0.5 and 5 phr; below theminimum indicated, the technical effect desired has proved inadequate,whereas beyond the maximum indicated there is the risk of impairing theprocessing in the uncured state of the compositions (for example,problem of solubility of the HMT) or the vulcanisation (slowed down inthe presence of H3M). For all these reasons, a quantity of between 0.5and 3.5 phr is more preferably selected, amounts lying within a rangefrom 1 to 3 phr being particularly advantageous in the case of treadsfor passenger-car tires.

Finally, the quantity of methylene donor, in the ranges indicated above,is advantageously adjusted so as to represent between 10% and 80%, morepreferably within a range from 40% to 60%, by weight relative to thequantity of methylene acceptor.

II-6. Various additives. Of course, the rubber compositions of thetreads according to the invention also comprise all or part of theadditives usually used in sulphur-cross-linkable diene rubbercompositions intended for the manufacturing of treads, such as, forexample, plasticisers, pigments, protective agents of the typeantioxidants, antiozonants, a cross-linking system based either onsulphur or on sulphur and/or peroxide and/or bismaleimide donors,vulcanisation accelerators, vulcanisation activators, extender oils,etc. There may also be associated with the reinforcing inorganic filler,if necessary, a conventional non-reinforcing white filler, such as forexample particles of clay, bentonite, talc, chalk, kaolin or titaniumoxides.

The rubber compositions of the treads of the invention may also contain,in addition to the coupling agents, agents for covering the reinforcinginorganic filler (comprising for example the single function Y), or moregenerally processing aids liable, in known manner, owing to animprovement in the dispersion of the inorganic filler in the rubbermatrix and to a reduction in the viscosity of the compositions, toimprove their ability to be worked in the uncured state, these agentsbeing, for example, alkylalkoxysilanes, (in particularalkyltriethoxysilanes), polyols, polyethers (for example polyethyleneglycols), primary, secondary or tertiary amines, hydroxylated orhydrolysable polyorganosiloxanes, for exampleα,ω-dihydroxypolyorganosiloxanes (in particularα,ω-dihydroxy-polydimethylsiloxanes).

II-7. Manufacturing of the treads. The rubber compositions of the treadsof the invention are manufactured in suitable mixers, using twosuccessive preparation phases in accordance with a general processwell-known to the person skilled in the art (see for example documentsEP-A-0 501 227 or WO00/05300 mentioned above): a first phase ofthermomechanical working or kneading (sometimes referred to as“non-productive” phase) at high temperature, up to a maximum temperatureof between 130° C. and 200° C., preferably between 145° C. and 185° C.,followed by a second phase of mechanical working (sometimes referred toas “productive” phase) at lower temperature, typically less than 120°C., for example between 60° C. and 100° C., during which finishing phasethe cross-linking or vulcanisation system is incorporated.

The process according to the invention, for preparing asulphur-vulcanizable tire tread, liable to have, after mechanicalrunning-in of the tire for which it is intended, a rigidity gradientradially increasing from the surface towards the inside of the tread,comprises the following steps:

-   -   incorporating in a diene elastomer, in a mixer:    -   more than 50 phr of an inorganic filler as reinforcing filler;    -   between 2 and 15 phr of an (inorganic filler/diene elastomer)        coupling agent;    -   between 1 and 10 phr of a methylene acceptor,    -   by thermomechanically kneading the entire mixture, in one or        more stages, until a maximum temperature of between 130° C. and        200° C. is reached;    -   cooling the entire mixture to a temperature of less than 100°        C.;    -   then incorporating:    -   between 0.5 and 5 phr of a methylene donor,    -   a vulcanisation system;    -   kneading the entire mixture until a maximum temperature of less        than 120° C.;    -   extruding or calendering the rubber composition thus obtained,        in the form of a tire tread.

According to a preferred embodiment of the invention, all the baseconstituents of the compositions of the treads according to theinvention, with the exception of the methylene donor and thevulcanization system, namely the reinforcing inorganic filler, thecoupling agent (and any activator), and the methylene acceptor areincorporated intimately by kneading in the diene elastomer during thefirst, so-called non-productive, phase, that is to say that at leastthese different base constituents are introduced into the mixer and arekneaded thermomechanically, in one or more stages, until the maximumtemperature of between 130° C. and 200° C., preferably between 145° C.and 185° C., is reached.

More preferably, during this first non-productive phase, the methyleneacceptor is incorporated in the mixer later than the elastomer, thereinforcing inorganic filler and its coupling agent, when thetemperature of the composition in the mixer, during kneading, hasreached a value of between 80° C. and 130° C., more preferably between90° C. and 110° C. In actual fact, better effectiveness of the M.A.D.system was noted for such temperature conditions.

By way of example, the first (non-productive) phase is effected in asingle thermomechanical step during which all the necessaryconstituents, any additional covering agents or processing agents andvarious other additives, with the exception of the methylene donor andthe vulcanization system, are introduced into a suitable mixer, such asa conventional internal mixer. A second stage of thermomechanicalworking may possibly be added, in this internal mixer, for example afteran intermediate cooling stage (preferably to a temperature of less than100° C.), with the aim of making the compositions undergo complementaryheat treatment, in particular in order to improve the dispersion, in theelastomeric matrix, of the reinforcing inorganic filler, the couplingagent and the methylene acceptor.

After cooling the mixture thus obtained during the first non-productivephase, the methylene donor and the vulcanisation system are thenincorporated at low temperature, generally in an external mixer such asan open mill; the entire composition is then mixed (productive phase)for several minutes, for example between 5 and 15 minutes.

The vulcanisation system proper is preferably based on sulphur and aprimary vulcanization accelerator, in particular an accelerator of thesulphenamide type. To this vulcanization system there are added,incorporated during the first non-productive phase and/or during theproductive phase, various known secondary accelerators or vulcanizationactivators such as zinc oxide, stearic acid, guanidine derivatives (inparticular diphenylguanidine), etc. The amount of sulphur is preferablybetween 0.5 and 3.0 phr, and the amount of the primary accelerator ispreferably between 0.5 and 5.0 phr.

The final composition thus obtained is then calendered, for example inthe form of a film or a sheet, in particular for characterisation in thelaboratory, or alternatively extruded in the form of a rubber profiledelement usable directly as a tire tread.

The vulcanization (or curing) is carried out in known manner at atemperature generally between 130° C. and 200° C., for a sufficient timewhich may vary, for example, between 5 and 90 minutes, depending, inparticular, on the curing temperature, the vulcanization system adoptedand the vulcanization kinetics of the composition in question.

In the process according to the invention, in accordance with thepreceding information given for the rubber compositions, preferably atleast one, more preferably all, of the following characteristics aresatisfied:

-   -   the quantity of reinforcing inorganic filler is between 60 and        120 phr;    -   the quantity of coupling agent is between 4 and 12 phr;    -   the quantity of methylene acceptor is between 2 and 8 phr;    -   the quantity of methylene donor is between 0.5 and 3.5 phr;    -   the maximum thermomechanical kneading temperature is between        145° C. and 180° C.;    -   the reinforcing inorganic filler is a siliceous or aluminous        filler;    -   the quantity of carbon black is between 2 and 15 phr;    -   the at least bifunctional coupling agent is an organosilane or a        polyorganosiloxane;    -   the methylene acceptor is selected from the group consisting of        phenolic resins;    -   the methylene donor is selected from the group consisting of        HMT, H3M, hexaethoxymethylmelamine, para-formaldehyde polymers,        N-methylol derivatives of melamine, or a mixture of these        compounds;    -   the quantity of methylene acceptor is between 2% and 15% by        weight relative to the weight of reinforcing inorganic filler;    -   the quantity of methylene donor represents between 10% and 80%        by weight relative to the weight of methylene acceptor.    -   the diene elastomer is a butadiene-styrene copolymer (SBR),        preferably used in a mixture with a polybutadiene;    -   the reinforcing inorganic filler represents more than 80% of the        total reinforcing filler.

More preferably, in this process, at least one, even more preferablyall, of the following characteristics are satisfied:

-   -   the quantity of inorganic filler lies within a range from 70 to        100 phr;    -   the quantity of coupling agent is between 3 and 8 phr;    -   the quantity of methylene acceptor lies within a range from 3 to        6 phr;    -   the quantity of methylene donor lies within a range from 1 to 3        phr;    -   the reinforcing inorganic filler is silica;    -   the quantity of carbon black is between 4 and 12 phr;    -   the coupling agent is a bis-(C₁–C₄)alkoxylsilylpropyl        polysulphide;    -   the methylene acceptor is a novolac phenolic resin;    -   the methylene donor is selected from the group consisting of        HMT, H3M or a mixture of these compounds;    -   the quantity of methylene acceptor is between 4% and 10% by        weight relative to the weight of reinforcing inorganic filler;    -   the quantity of methylene donor represents from 40% to 60% by        weight relative to the weight of methylene acceptor;    -   the SBR is an SBR prepared in solution and the polybutadiene has        more than 90% cis-1,4 bonds;    -   the reinforcing inorganic filler represents more than 90% of the        total reinforcing filler.

The rubber compositions previously described, based on diene elastomer,a high amount of reinforcing inorganic filler, a coupling agent and amethylene acceptor/donor system, in the proportions indicated above, mayadvantageously constitute the entire tread according to the invention.

However, the invention also applies to those cases in which these rubbercompositions comprising the M.A.D. system form only part of a compositetread such as described for example in the introduction to the presentspecification, formed of two radially superposed layers of differentrigidity (so-called “cap-base” structure), both intended to come intocontact with the road during running of the tire, during the life of thelatter.

The part based on the M.A.D. system may then constitute the radiallyouter layer of the tread which is intended to come into contact with theground from the start of running of the new tire, or on the contrary itsradially inner layer which is intended to come into contact with theground later, in those cases in which it is desired, for example, to“retard” the technical effect of auto-accommodation provided by theinvention.

Of course, the invention relates to the treads previously described,both in the uncured state (i.e. before curing) and in the cured state(i.e. after cross-linking or vulcanization).

III. Examples of Embodiment of the Invention

III-1. Preparation of the rubber compositions and treads. For thefollowing tests, the procedure is as follows: the reinforcing filler,the coupling agent and any associated coupling activator, the dieneelastomer or the mixture of diene elastomers, the methylene acceptor andthe various other ingredients, with the exception of the vulcanizationsystem and the methylene donor, are introduced in succession into aninternal mixer filled to 70% of capacity, the initial tank temperatureof which is approximately 60° C. Thermomechanical working(non-productive phase) is then performed in one stage, of a duration ofabout 3 to 4 minutes in total, until a maximum “dropping” temperature of165° C. is obtained. In these tests, the methylene acceptor isintroduced into the mixer when the composition under kneading hasreached a temperature close to 100° C.

The mixture thus obtained is recovered, it is cooled and then thesulphur, sulphenamide accelerator and methylene donor are incorporatedon an external mixer (homo-finisher) at 30° C., by mixing everything(productive phase) for an appropriate time, of between 5 and 12 minutesdepending on the case.

The compositions thus obtained are then calendered either in the form ofplates (thickness of 2 to 3 mm) or of thin sheets of rubber in order tomeasure their physical or mechanical properties, or extruded in the formof treads for passenger-car tires.

In all the tests which follow, the reinforcing inorganic filler (silicatype “HD”) is present in an amount greater than 60 phr; it furthermoreconstitutes more than 90% by weight of all the reinforcing filler, aminority fraction (less than 10%) of the latter being constituted bycarbon black.

III-2. Tests. A) Test 1. In this first test, four rubber compositionsare compared, based on known SBR and BR diene elastomers, reinforced bycarbon black or silica, used as treads for tires for passenger vehicles.

These four compositions C-1 to C-4 are distinguished essentially by thefollowing characteristics:

-   -   C-1: reinforced with carbon black (70 phr), without M.A.D.        system;    -   C-2: reinforced with carbon black (70 phr), with M.A.D. system;    -   C-3: reinforced with silica (80 phr), without M.A.D. system;    -   C-4: reinforced with silica (80 phr), with M.A.D. system.

The compositions C-1 and C-3 constitute the controls “black” and“silica” of this test. Their respective formulations were adjusted to asto bring them both to initial iso-rigidity (Shore A hardness and modulusMIO equivalent), before incorporation of the M.A.D. system.

Composition C-3 furthermore contains the TESPT coupling agent (amount of8% by weight relative to the quantity of silica) and the DPG (about 2.6%by weight relative to the quantity of silica). In this composition C-3,the carbon black is essentially used as black pigmentation agent, and ispresent in a very small amount (6 phr, or approximately 7% of the totalreinforcing filler).

Compositions C-2 and C-4 correspond respectively to compositions C-1 andC-3 to which the M.A.D. system has been added; only the tread comprisingcomposition C-4 is therefore in accordance with the invention.

Tables 1 and 2 show the formulation of the different compositions (Table1-amounts of the different products expressed in phr), and theirproperties before and after curing (40 min at 150° C.).

Comparison, first of all, of the control compositions C-1 and C-3(devoid of M.A.D. system) results in the following observations:

-   -   in the uncured state, a lower Mooney viscosity of composition        C-2 based on silica and a slightly increased scorching time,        which is beneficial to the processing of the composition;    -   after curing, an equivalent rigidity (identical Shore A        hardnesses; moduli at low deformation M10 very close) and        identical reinforcement properties (identical moduli at the high        deformations (M100 and M300), equivalent mechanical properties        at break, except for the accuracy of measurement);    -   finally, expectedly, a lower hysteresis (lower values of ΔG* and        tan(δ)_(max)) for composition C-2 based on silica, which is        synonymous with a lower rolling resistance.

After incorporation of the M.A.D. system, the following modifications ofproperties are observed for the two types of compositions (compare C-2with C-1 on one hand, and C-4 with C-3 on the other hand):

-   -   in the uncured state, a Mooney viscosity which is little        modified, a reduction in T5 in both cases (C-2 and C-4), with        values T5 (at least 10 min) which nevertheless offer a        sufficient safety margin with respect to the problem of        scorching;    -   a substantial increase in the hysteresis in both cases; it will        however be noted that the values of ΔG* and tan(δ)_(max)        measured on the composition based on silica (C-4) remain at a        low level, substantially equal to that of the control        composition C-1 filled with carbon black, which constitutes a        significant advantage for the silica-based tread;    -   finally, a great rise in the value of modulus at low deformation        (values M10 practically doubled) and in the Shore hardness        (increased by about 20%).

These results may be regarded as expected. In particular, the verydistinct increase in rigidity (Shore hardness and modulus at lowdeformation M10), due to the presence of the M.A.D. system in thesecompositions C-2 and C-4, enables the person skilled in the art toexpect, for tires mounted on automobiles the treads of which are formedof such compositions, certainly an improvement in the road behaviourowing to an increased drift thrust, but above all a crippling drop inthe grip performance on wet ground.

However, one significant difference must be noted between compositionsC-2 and C-4, this difference relating to the evolution of the modulus(M10_(Ac)) at low deformation, after mechanical accommodation (15%).

In the case of the control composition C-2 (carbon black), it will benoted that the modulus M10_(Ac) remains very high after accommodation(9.1 MPa compared with an initial 5.5 MPa on composition C-1, orapproximately 65% greater); on the contrary, this same modulus M10_(Ac)drops very greatly (from 12.0 MPa to 7.2 MPa) for composition C-4(silica), recovering practically the initial value M10 (6.0 MPa)recorded for the control composition C-3 devoid of M.A.D. system.

Such a difference in response between the two compositions, in thepresence of the M.A.D. system, is totally unexpected; it justifies thesefirst results now facing real tire running tests, as set forth in Test 2below.

B) Test 2 (running tests for tires). Compositions C-1 to C-4 previouslydescribed are used in this test as treads for radial carcasspassenger-car tires, of dimension 195/65 R15 (speed index H),conventionally manufactured and identical in all points except for therubber composition constituting the tread.

These tires are marked P-1 to P-4 and correspond to compositions C-1 toC-4, respectively; they were first tested on a machine to determinetheir initial Shore A hardness (on tread) and their drift thrust.

They were then mounted on a passenger vehicle in order to be subjectedto the braking and grip tests described in section I-6 above, inaccordance with the following specific conditions:

-   -   braking on damp ground: vehicle Renault model Laguna (front and        rear pressure: 2.0 bar), the tires to be tested being mounted at        the front of the vehicle;    -   travel on a damp circuit comprising bends: vehicle BMW model 328        (front pressure 2.0 bar; rear pressure: 2.4 bar), the tires to        be tested being mounted at the front and at the rear of the        vehicle.

Finally, their Shore A hardness was also measured after “standardrunning-in” (see (C) of section I-6) on the Renault Laguna vehicle.

The results obtained, set forth in Table 3, result in the followingcomments:

-   -   it will be noted first of all that the Shore A hardnesses        measured right at the surface of the treads of the new tires        (“initial” Shore A hardnesses) are virtually equal to those        measured on rubber test pieces (corresponding compositions C-1        to C-4—see Table 2 above), whatever the type of reinforcing        filler used (carbon black or silica); the addition of the M.A.D.        system results in both cases in a distinct increase in Shore        hardness, of about 13 points (from 67 points to 79–80 points);    -   this increase in rigidity of the tread, for the two types of        tires P-2 and P-4, is accompanied predictably by a very great        increase in the drift thrust (+30%), which is a clear indicator        to the person skilled in the art of improved road behaviour (on        dry ground);    -   as far as the braking performance on damp ground is concerned,        it will be noted first of all that the control tires P-1 and        P-3, which are devoid of the M.A.D. system, exhibit an        equivalent performance (base 100 used for the tire P-1);    -   after incorporation of the M.A.D. system, the control tire P-2,        the tread of which is reinforced with carbon black, exhibits a        crippling drop (20% loss) in this braking performance, which was        furthermore entirely predictable, taking into account the great        stiffening of the tread provided by the M.A.D. system;    -   the unexpected result lies in the behaviour of the tire of the        invention P-4, the tread of which is reinforced with silica: not        only is its braking performance on wet ground not degraded—which        is already a very surprising result for the person skilled in        the art—but it is substantially improved since an improvement of        8% is obtained relative to the control tires P-1 and P-3; this        behaviour is entirely noteworthy and radically opposed to that        of the control tire P-2;    -   as for the running test on a damp circuit comprising bends, it        confirms that the incorporation of the M.A.D. system in the        conventional tread filled with carbon black (tire P-2) results        in an unacceptable drop in grip, which is illustrated both by an        increase in the minimum time necessary to travel round the        circuit at limit speed conditions (plus 7%) and by a reduction        in the behaviour mark attributed by the driver (drop of 25%,        which is very significant for such a test);    -   whereas the incorporation of the same M.A.D. system in the tread        of the invention filled with silica (tire P-4) results on the        contrary in a very significant improvement in performance: time        to travel round the circuit significantly reduced (improvement        of 3%) compared with the control tires, better behaviour mark        (improvement of 10%);    -   correlatively to the running results above, it will be noted        that after mechanical accommodation (“standard running-in”), the        Shore A surface hardness remains unchanged (except for the        accuracy of measurement) on the control tires P-1, P-2 and P-3,        whereas it drops very significantly (less 10 points) on the tire        P-4 according to the invention; this remarkable behaviour        furthermore recalls the evolution of the modulus M10_(Ac)        observed on the corresponding rubber compositions (C-1 to C-4)        in Test 1 above.

In summary, the tire according to the invention P-4, entirelyunexpectedly, exhibits a simultaneous increase in two properties whichhowever have been deemed contradictory, namely road behaviour (driftthrust) and grip on wet ground.

It must be deduced from this that, necessarily, the three-dimensionalresin lattice provided by the M.A.D. system in the rubber compositionsof the treads is “expressed” differently depending on whether thesecompositions are filled conventionally with carbon black, or rather witha reinforcing inorganic filler such as silica, in the recommended highamount.

C) Test 3. This third test, a posteriori, provides an explanation forthe improved results of Tests 1 and 2 above, by revealing an unexpectedproperty for the tread of the invention: the latter in fact has, in theradial direction, a very marked rigidity gradient, with an increasingrigidity when moving from the surface towards the inside of the tread;such a characteristic does not exist in the control tread filled withcarbon black.

FIG. 1 represents, for the treads of tires P-2 and P-4 of the previoustest, the evolution of modulus M10 (in MPa) as a function of the depth“e” (from 0 to 6 mm), before (as-new state) and after “standardrunning-in” of these tires on the aforementioned Renault Laguna vehicle.

In order to obtain these rigidity profiles, strips of tread were cutout, at regular intervals of depth “e” (for example every 2/10 mm),practically over the entire thickness of this tread (approximately 7mm); then these strips were subjected to traction to determine theirmodulus M10 as a function of their depth “e” in the tread, measuredrelative to the surface (e=0 mm) of the latter.

More precisely, there are the following correspondences:

-   -   the curve A corresponds to the control tires P-1 and P-3, that        is to say to the treads without M.A.D. system, the profiles of        modulus essentially coinciding (except for the accuracy of        measurement) between the two types of reinforcing filler used        (carbon black or silica);    -   the curve B corresponds to the tires P-2 and P-4, that is to say        to the treads with M.A.D. system, these tires being new, that is        to say before any mechanical running-in or accommodation; here        too, the profiles of modulus essentially coincide between carbon        black and silica;    -   the curve C corresponds to the tire P-2 (carbon black) after        running-in;    -   the curve D corresponds to the tire P-4 (silica) after        running-in.

After running-in, radically different behaviour is observed between thetires P-2 and P-4:

-   -   the surface rigidity and the rigidity at depth are little        different for the tire P-2 of the prior art, the reinforcing        filler of which is carbon black;    -   whereas, in the case of the tire P-4 of the invention, the        surface rigidity is very significantly less, practically equal        to that of the control tires P-1 and P-3, with furthermore a        very marked radial rigidity gradient, which increases on moving        from the surface of the tread towards the inside of the latter.

It should perhaps be deduced from the curves A to D above, and from allthe above results, that the three-dimensional stiffening lattice formedby the M.A.D. system has lower solidity in the case of the tread filledwith silica than in the case of the conventional tread filled withcarbon black.

Due to this relative fragility, stresses of low amplitude, typical ofthose experienced during running by the surface part of the tread, wouldbe sufficient to break the surface resin lattice, and thus to make thesurface part of the tread more flexible and less rigid, and thus make itrecover the excellent grip performance which it has in the absence ofthe M.A.D. system. On the other hand, in depth, the lattice resin wouldbe little affected by running, all the more so as one penetrates insidethis tread, thus guaranteeing the additional rigidity sufficient forimproved road behaviour (greater drift thrust).

D) Test 4. In this new test, five rubber compositions are compared, allreinforced with silica (80 phr) and a small proportion (6 phr) of carbonblack as black pigmentation agent, these compositions also being used astreads for tires for passenger vehicles.

The control composition (C-5) is devoid of M.A.D. system, whereas theother four (C-6 to C-9) comprise such a system; the methylene acceptoris formed by different variants of novolac resins (6 phr), the methylenedonor being HMT (2 phr). Each composition comprises in particular acoupling agent for the silica. The treads comprising the compositionsC-6 to C-9 are therefore all in accordance with the invention. Tables 4and 5 show the precise formulation of the different compositions (Table4—amounts of the different products expressed in phr), and theirproperties before and after curing (40 min at 150° C.).

On reading these results, it will be noted that, compared with thecontrol composition C-5 which is devoid of M.A.D. system, thecompositions used as treads according to the invention have thefollowing characteristics:

-   -   in the uncured state, a lower Mooney viscosity, which is        beneficial to processing; a reduction in T5 in all cases, with        values (10 to 13 min) which nevertheless offer a sufficient        safety margin with respect to the problem of scorching;    -   after curing, reinforcement properties which are substantially        equivalent, illustrated by values close to the moduli at high        deformations (M100 and M300), and also the properties at break;    -   significantly greater rigidity, illustrated both by a Shore A        hardness increased by approximately 16% (change from 67 points        to an average value of 78 points) and by a modulus M10 at low        deformation which is substantially doubled (from 6.3 MPa to 12.5        MPa, on average);    -   correlatively, an expected increase in the hysteresis (increase        in dynamic properties ΔG* and tan(δ)_(max));    -   finally, after mechanical accommodation, values of modulus at        low deformation (M10_(Ac)) which drop very greatly, recovering        values closed to the initial value observed on the control        composition C-5 which is devoid of resin lattice.

The unexpected results of Test 1 above (virtual “reversibility” of themodulus M10, after mechanical accommodation) are thus clearly confirmed,in the presence of various types of novolac resins, making it possibleto predict, for tires comprising these compositions C-6 to C-9 astreads, the same improved compromise of road behaviour and grip as thatobserved in Test 2 above, owing to the presence of a radial rigiditygradient in the tread.

It will furthermore be noted that the compositions C-6 to C-9 accordingto the invention have very close properties, whatever the novolac resinused as methylene acceptor.

E) Test 5. Here five compositions filled with silica (C-10 to C-14) arecompared, similar to those of Test 4 above.

A first control composition (C-10) is devoid of M.A.D. system, a secondcontrol composition (C-11) comprises the methylene acceptor but not themethylene donor. The other three compositions (C-12 to C-14) constitutethree new variants, with different M.A.D. systems, of compositionsusable as treads according to the invention; it will be noted inparticular that composition C-14 comprises, as M.A.D. system, twodifferent methylene acceptors and two different methylene donors.

Tables 6 and 7 show the formulation of the different compositions (Table6—amounts of the different products expressed in phr), and theirproperties before and after curing (40 min at 150° C.).

Reading these results entirely confirms, if it were necessary, theconclusions of the above tests, namely, for the compositions C-12, C-13and C-14 according to the invention, compared with the control C-10:

-   -   in the uncured state, a lower Mooney viscosity, which is        beneficial to processing;    -   admittedly a reduction in T5, but values (15 min) which are        satisfactory with respect to the problem of scorching;    -   after curing, an (expected) increase in the hysteresis (ΔG* and        tan(δ)_(max));    -   reinforcement properties which are at least equal (values close        to the moduli M100 and M300; equivalent properties at break);    -   distinctly increased rigidity: plus 10 points on the Shore        hardness, modulus M10 virtually doubled (from 6.3 MPa to 10.8        MPa on average);    -   finally, after accommodation, values of modulus at low        deformation (M10_(Ac)) which practically recover those of the        control C-10.

In accordance with the results of the preceding tests, these latter twocharacteristics must be analysed as synonymous of improved roadbehaviour (due to the stiffening in depth of the tread) withoutadversely affecting or even improving the grip on wet ground (due to asurface rigidity which is virtually not modified, after accommodation).

It will be noted finally that the control composition C-11, comprising amethylene acceptor without curing agent, reveals intermediate propertieswhich are of little interest.

As for the composition C-14 according to the invention, comprising twodifferent methylene acceptors (formo-phenol+diphenylolpropane)associated with two methylene donors (HMT and H3M), it exhibits the bestoverall compromise of properties for this test, both before curing(viscosity and scorching time) and after curing (rigidity, reinforcementand hysteresis).

F) Test 6. In this test two new rubber compositions based on silica(C-15 and C-16) are compared which constitute two new variants ofcompositions usable as treads according to the invention. The amount ofreinforcing inorganic filler has been slightly reduced compared with thepreceding tests, while remaining within a preferred range of between 60phr and 100 phr.

The composition C-16 is identical to composition C-15, except that acoupling activator system such as described in the aforementionedapplication WO00/05301, formed by the association of a zincdithiophosphate (DTPZn) and a guanidine derivative (DPG) has been addedto C-16; such an activator has the ability to improve the effectivenessof a polysulphurised alkoxysilane coupling agent.

Tables 8 and 9 show the formulation of the different compositions, andtheir properties before and after curing (40 min at 150° C.).

On reading these results, it will be noted that the incorporation ofDTPZn and DPG in composition C-16 has a beneficial effect on the majorpart of the properties, with in particular:

-   -   an increase in the reinforcement, as indicated by a substantial        increase in the values M100 and M300, and in the ratio        M300/M100;    -   advantageously combined with a substantial reduction in the        hysteresis (reduction of ΔG* and tan(δ)_(max)).

The two evolutions above clearly illustrate a better interaction betweenthe reinforcing inorganic filler and the elastomer, in other words acoupling effect which is optimised due to the presence of the couplingactivator system.

It will furthermore be noted that this optimised reinforcement isobtained without adversely affecting the action of the M.A.D. system(values of Shore A hardness, moduli M10 and M10_(Ac) identical).

In the preceding results a slight increase in the hysteresis in thepresence of the M.A.D. system had been noted, for the compositions andtreads of the invention filled with silica, which is harmful withrespect to the rolling resistance. The use of a coupling activator is away of overcoming this drawback owing to the possibility which itoffers, for example, of reducing the amount of reinforcing inorganicfiller.

G) Test 7. Five rubber compositions are compared, all reinforced withsilica (80 phr) and a small proportion (6 phr) of carbon black, thesecompositions being used as treads for tires for passenger vehicles.

The control composition (C-17) is devoid of M.A.D. system. The other 4compositions (C-18 to C-21) comprise such a system; the methyleneacceptor is used therein in amounts varying from 2 to 5 phr, thequantity of methylene donor being selected to be equal to 60% by weightrelative to the quantity of acceptor.

Tables 10 and 11 give the formulation of the different compositions,some of their properties after curing (40 min at 150° C.), and theproperties of the tires (respectively P-17 to P-21) comprising thecorresponding treads. These tires are manufactured and tested asindicated previously for Test 2.

The treads comprising the compositions C-18 to C-21 and the tires P-18to P-21 are therefore all in accordance with the invention.

On reading the results of Table 11, it will be noted first of all that,compared with the control composition C-17, the compositions C-18 toC-21 have, all the more so the larger the quantity of M.A.D. system, agreater rigidity illustrated by a Shore A hardness and a modulus M10both of which are significantly increased, the modulus M10 beingsubstantially doubled (from 5.6 MPa to 11.3 MPa) for the highest amountof M.A.D. system.

This greater rigidity results, for the tires according to the invention,not only in an (expected) increase in the drift thrust (an indicator ofthe behaviour on dry ground), but also and in particular, unexpectedly,in an improvement in the behaviour on wet ground (see performance on adamp circuit comprising bends).

There are thus confirmed, in the presence of the M.A.D. system combinedwith the high amount of reinforcing inorganic filler, the improvedoverall compromise of road behaviour and grip as observed in Test 2above, obtained due to the presence of a radial rigidity gradient in thetreads according to the invention.

H) Test 8. In this Test 2 new compositions are compared which arereinforced with a high amount of silica, used to constitute all or of atread for passenger-vehicle tires.

The composition according to the invention (C-23) comprises the M.A.D.system, whereas the control composition (C-22) is devoid thereof. Theformulation of the 2 compositions is close to those of Table 6 (Test 5),except that, in particular, the diene elastomer used here is a mixtureof two SBRs of different microstructures.

Tables 12 and 13 give this detailed formulation of the 2 compositions,some of their properties after curing (40 min at 150° C.), and theproperties of the corresponding tires (respectively P-22 and P-23),manufactured and tested as indicated for Test 2 above, except for thefollowing details:

the tread of the control tires P-22 is formed exclusively by the controlcomposition C-22;

the tread of the tires P-23 according to the invention is a compositetread having a “cap/base” structure such as described above, formed bythe two radially superposed compositions C-22 and C-23, the compositionaccording to the invention C-23 constituting the base, that is to saythe radially inner part (on a new tire) of the tread.

On reading the results of Table 13, it will be noted first of all thatthe presence of the M.A.D. system in the composition C-23 involves avery substantial increase in Shore A hardness (nearly 20% greater). Thisgreater rigidity results, for the tires P-23 according to the invention,in significantly improved on-vehicle behaviour, not only on dry ground(increase in the drift thrust) but also and in particular on wet ground:travelling time distinctly reduced (less than 4 seconds approximatelyper lap of the circuit) and a significantly improved behaviour mark(+15%).

Thus, the beneficial effect of the invention, obtained due to thephenomenon of auto-accommodation, is once again demonstrated, even inthe case in which the composition based on the M.A.D. system constitutesonly part of the tread, in this case in this test the radially innerpart of the tread which is intended to come into contact with the roadonly later, after its (radially) outer part of this tread has becomeworn.

I) Test 9. In this last test, 2 new compositions reinforced with silicaare compared, which are similar to those of Test 1 (compositions C-3 andC-4) except that, in particular, in this case an HD silica of smallerspecific surface area is used. The composition according to theinvention (C-25) comprises the M.A.D. system, whereas the controlcomposition (C-24) is devoid thereof; these 2 compositions are used astreads for passenger-car tires.

Tables 14 and 15 give the formulation of the compositions, some of theirproperties before and after curing (40 min at 150° C.), and theproperties of the corresponding tires (respectively P-24 and P-25)(manufactured and tested as indicated for Test 2 above).

The results of Table 15 confirm once again the beneficial unexpectedeffects of the invention (composition C-25 and tires P-25), due to thecombined presence of the M.A.D. system and the high amount ofreinforcing inorganic filler, with:

a significant reduction in viscosity in the uncured state;

a very substantial increase in the rigidity in the cured state (Shore Ahardness and modulus M10), without adversely affecting the level ofreinforcement (illustrated by the modulus M100);

finally, once again, significantly improved behaviour of the tiresaccording to the invention, both on dry ground (drift thrust increasedby 15%) and on wet ground, due to the auto-accommodation resulting fromthe combined presence of the M.A.D. system and a high amount ofreinforcing inorganic filler.

In conclusion, due to the treads according to the invention and to thespecific formulation of their rubber compositions, it is henceforthpossible to “reconcile” what had hitherto appeared irreconcilable,namely grip on one hand and road behaviour on the other hand, withoutusing solutions which are complex, costly or non-durable such asdescribed in the introduction to the present specification.

The treads according to the invention described in the precedingexamples offer the major advantage, compared with the “composite” treadsof the prior art, on one hand of maintaining their compromise ofperformances throughout the life of the tire, due to the unexpectedphenomenon of auto-accommodation, and on the other hand of having a trueradial rigidity gradient, and not a simple, very localised, “accident”of rigidity. This true rigidity gradient results in optimum “working” ofthe blocks of rubber in contact with the ground, during running and thenumerous forces transmitted to the tread, in other words is synonymouswith a tire which grips the road even better.

The invention will find a very advantageous application in tires fittedon vehicles such as motorcycles, passenger cars, vans or heavy vehicles,in particular in high-grip tires of the “snow” or “ice” type (alsoreferred to as “winter” tires) which, owing to a deliberately moreflexible tread, could hitherto have qualities of road behaviour, on dryground, which were sometimes judged to be insufficient.

The novel compromise of properties thus obtained, which has clearlyshifted compared with the acquired knowledge of the prior art, mayfurthermore be obtained while keeping the performances of rollingresistance and of wear resistance at the high levels which one isentitled to expect nowadays of rubber compositions based on reinforcinginorganic fillers such as highly dispersible silicas, capable ofreplacing the conventional tire-grade carbon blacks.

TABLE 1 Composition No.: C-1 C-2 C-3 C-4 SBR (1) 70 70 70 70 BR (2) 3030 30 30 carbon black (3) 70 70 6 6 silica (4) — — 80 80 coupling agent(5) — — 6.4 6.4 methylene acceptor (6) — 6 — 6 oil (7) 20 20 39 39 DPG(8) — — 2.1 2.1 ZnO 2.5 2.5 2.5 2.5 stearic acid 2 2 2 2 antioxidant (9)1.9 1.9 1.9 1.9 methylene donor (10) — 2 — 2 sulphur 1.5 1.5 1.5 1.5accelerator (11) 1.5 1.5 1.5 1.5 (1) SBR (expressed in dry SBR) extendedwith 37.5% by weight (26.25 phr) of oil (or a total of 96.25 phr ofextended SBR); 26.5% styrene, 59.5% 1-2-polybutadiene units and 23%trans-1-4-polybutadiene units (Tg = −29° C.); (2) BR with 4.3% of 1–2;2.7% of trans; 93% of cis 1–4 (Tg = −106° C.); (3) carbon black N234;(4) silica “Zeosil 1165 MP” from Rhodia, type “HD” (BET and CTAB:approximately 160 m²/g); (5) TESPT coupling agent (“Si69” from Degussa);(6) formo-phenol novolac resin (“Peracit 4536K” from Perstorp); (7)total aromatic oil (including extender oil for the SBR); (8)diphenylguanidine (Perkacit DPG from Flexsys); (9)N-1,3-dimethylbutyl-N-phenylparaphenylenediamine (Santoflex 6-PPD fromFlexsys); (10) HMT (from Degussa); (11) N-cyclohexyl-2-benzothiazylsulphenamide (Santocure CBS from Flexsys).

TABLE 2 Composition No.: C-1 C-2 C-3 C-4 Properties before curing:Mooney (MU) 100 105 75 70 T5 (min) 15 10 19 13 Properties after curing:M10 (MPa) 5.5 11.8 6.0 12.0 M100 (MPa) 1.85 2.3 1.9 2.2 M300 (MPa) 2.22.0 2.15 1.6 Shore A 65 79 66 80 ΔG* 10 17 6.8 10 tan(δ)_(max) 0.38 0.430.32 0.39 M10_(Ac) 4.4 9.1 4.8 7.2 Breaking stress (MPa) 20 19 19 15Elongation at break (%) 590 550 540 620

TABLE 3 tires: P-1 P-2 P-3 P-4 Initial Shore A hardness:  67  79  67  80(evolution in r.u.) (100) (118) (100) (119) Drift thrust (r.u.): 100 130100 130 Braking on damp ground 100  80 100 108 (r.u.): Performance on adamp circuit comprising bends (r.u.): travelling time: 100 107 100  97behaviour mark: 100  75 100 110 Shore A hardness after  67  78  66  70“standard running-in”: (100) (116) (99) (106) (evolution in r.u.)

TABLE 4 Composition No.: C-5 C-6 C-7 C-8 C-9 SBR (1) 60 60 60 60 60 BR(2) 40 40 40 40 40 carbon black (3) 6 6 6 6 6 silica (4) 80 80 80 80 80coupling agent (5) 6.4 6.4 6.4 6.4 6.4 methylene acceptor (6) — 6 6 6 6oil (7) 39 39 39 39 39 DPG (8) 1.5 1.5 1.5 1.5 1.5 ZnO 2.5 2.5 2.5 2.52.5 stearic acid 2 2 2 2 2 antioxidant (9) 1.9 1.9 1.9 1.9 1.9 methylenedonor (10) — 2 2 2 2 sulphur 1.1 1.1 1.1 1.1 1.1 accelerator (11) 2 2 22 2 (1) to (5): idem Table 1; (6): novolac resins: C-6: idem Table 1;C-7: modified alkylphenol (XR14545C from Ceca); C-8: modified “tallol”(tall oil) (XR146212C from Ceca); C-9: formophenolic (ortho catalysis)(XR4364 from Ceca); (7) to (11): idem Table 1.

TABLE 5 Composition No.: C-5 C-6 C-7 C-8 C-9 Properties before curing:Mooney (MU) 93 81 74 76 79 T5 (min) 18 12 10 13 11 Properties aftercuring: M10 (MPa) 6.3 12.4 13.7 12.0 11.6 M100 (MPa) 1.8 2.1 2.1 2.1 2.1M300 (MPa) 2.0 1.8 1.8 1.85 1.85 Shore A 67 78 79.5 77 77 ΔG* 6.1 12.411.8 12.7 12.0 tan(δ)_(max) 0.32 0.37 0.38 0.39 0.38 M10_(Ac) 4.9 7.58.2 7.1 7.1 Breaking stress (MPa) 21 18 19 18 17 Elongation at break (%)595 610 630 575 560

TABLE 6 Composition No.: C-10 C-11 C-12 C-13 C-14 SBR (1) 70 70 70 70 70BR (2) 30 30 30 30 30 carbon black (3) 6 6 6 6 6 silica (4) 80 80 80 8080 coupling agent (5) 6.4 6.4 6.4 6.4 6.4 methylene acceptor (6) — 5 5 51.5 methylene acceptor (6 bis) — — — — 2.5 oil (7) 33 33 33 33 33 DPG(8) 1.5 1.5 1.5 1.5 1.5 ZnO 2.5 2.5 2.5 2.5 2.5 stearic acid 2 2 2 2 2antioxidant (9) 1.4 1.4 1.4 1.4 1.4 methylene donor (10) — — — 1.5 0.75methylene donor (12) — — 1.5 — 1.5 sulphur 1.5 1.5 1.5 1.5 1.5accelerator (11) 1.5 1.5 1.5 1.5 1.5 (1) to (6) idem Table 1; (6 bis)diphenylolpropane (“Bisphenol A” from Bayer); (7) to (11) idem Table 1;(12) H3M (“Additol VXT 3923” from Vianova resins).

TABLE 7 Composition No.: C-10 C-11 C-12 C-13 C-14 Properties beforecuring: Mooney (MU) 90 91 84 85 78 T5 (min) 17 11 14 14 14 Propertiesafter curing: M10 (MPa) 6.3 9 12 11.2 11 M100 (MPa) 1.9 2.0 2.5 2 2.2M300 (MPa) 2.1 2.0 2.3 1.7 2.1 Shore A 68 72 78.5 78 77 ΔG* 6.8 11 1512.4 12.9 tan(δ)_(max) 0.34 0.41 0.40 0.37 0.40 M10_(Ac) 5.0 6.6 7.3 6.96.7 Breaking stress (MPa) 19 20 18 17 18 Elongation at break (%) 540 585510 600 550

TABLE 8 Composition No.: C-15 C-16 SBR (1) 70 70 BR (2) 30 30 carbonblack (3) 6 6 silica (4) 70 70 coupling agent (5) 5.6 5.6 methyleneacceptor (6) 4 4 oil (7) 25 25 DTPZn (10) — 0.75 DPG (8) 1.3 1.3 ZnO 2.52.5 stearic acid 2 2 antioxidant (9) 1.9 1.9 methylene donor (12) 1 1sulphur 1.5 1.5 accelerator (11) 1.5 1.5 (1) to (9) idem Table 1; (10)Rhenocure TP/G manufactured by Rhein-Chemie (50% by weight of DTPZn onelastomeric support - or, here, 1.5 phr of Rhenocure TP/G); (11)–(12)idem Table 5.

TABLE 9 Composition No.: C-15 C-16 Properties before curing: Mooney (MU)88 88 T5 (min) 16 10 Properties after curing: M10 (MPa) 9.2 9 M100 (MPa)2.1 2.5 M300 (MPa) 2.1 2.9 Shore A 74 74.5 G* 10 8 tan(δ)_(max) 0.350.32 M10_(Ac) 5.7 5.6 Breaking stress (MPa) 19 17 Elongation at break(%) 585 485

TABLE 10 Composition No.: C-17 C-18 C-19 C-20 C-21 SBR (1) 70 70 70 7070 BR (2) 30 30 30 30 30 carbon black (3) 6 6 6 6 6 silica (4) 80 80 8080 80 coupling agent (5) 6.4 6.4 6.4 6.4 6.4 methylene acceptor (6) — 54 3 2 oil (7) 33 33 33 33 33 DPG (8) 1.5 1.5 1.5 1.5 1.5 ZnO 2.5 2.5 2.52.5 2.5 stearic acid 2 2 2 2 2 antioxidant (9) 1.4 1.4 1.4 1.4 1.4methylene donor (10) — 3 2.4 1.8 1.2 sulphur 1.1 1.1 1.1 1.1 1.1accelerator (11) 2.0 2.0 2.0 2.0 2.0 (1) to (11) idem Table 1.

TABLE 11 Composition No. C-17 C-18 C-19 C-20 C-21 M10 (MPa) 5.6 11.3 9.98.6 7.8 Shore A 70 82 79 77 74 tire No.: P-17 P-18 P-19 P-20 P-21 Driftthrust (r.u.) 100 122 116 111 109 Performance on a damp circuitcomprising bends (r.u.): travelling time: 100 98.5 99.2 98.4 99.5behaviour mark: 100 115 115 115 105

TABLE 12 Composition No. C-22 C-23 SBR (1) 55 55 SBR (2) 45 45 carbonblack (3) 6 6 silica (4) 82 82 coupling agent (5) 6.6 6.6 methyleneacceptor (6) — 1.5 methylene acceptor (6 bis) — 2.5 oil (7) 41.5 41.5DPG (8) 1.5 1.5 ZnO 2.5 2.5 stearic acid 2 2 antioxidant (9) 1.9 1.9methylene donor (10) — 0.75 methylene donor (12) — 1.5 sulphur 1.4 1.4accelerator (11) 1.5 1.5 (1) SBR (expressed in dry SBR) extended with37.5% (20.6 phr) of oil (or a total of 75.6 phr of extended SBR); 27%stirene, 27% 1-2-polybutadiene units and 78% trans-1-4-polybutadieneunits (Tg = −50° C.); (2) SBR (expressed in dry SBR) extended with 37.5%(16.9 phr) of oil (or a total of 61.9 phr of extended SBR); 40% stirene,25% 1-2-polybutadiene units and 48% trans-1-4-polybutadiene units (Tg =−30° C.); (3) to (12) idem Table 6.

TABLE 13 Composition No. C-22 C-23 Shore A  72  82 (evolution in r.u.)(100) (119) tire No.: P-22 P-23 Drift thrust (r.u.): 100 125 Performanceon a damp circuit comprising bends (r.u.): travelling time: 100  96behaviour mark: 100 115

TABLE 14 Composition No. C-24 C-25 SBR (1) 70 70 BR (2) 30 30 carbonblack (3) 6 6 silica (4) 80 80 coupling agent (5) 4.8 4.8 methyleneacceptor (6) — 5 oil (7) 33 33 DPG (8) 1.1 1.1 ZnO 2.5 2.5 stearic acid2 2 antioxidant (9) 1.9 1.9 methylene donor (10) — 3 sulphur 1.1 1.1accelerator (11) 2 2 (4) silica “Zeosil 1115 MP” from Rhodia, type “HD”(BET and CTAB: approximately 110 m²/g); (1) to (3) and (5) to (11): idemTable 1.

TABLE 15 Composition No. C-24 C-25 Mooney (MU) 71 64 M10 (MPa) 5.5 10.9M100 (MPa) 1.8 1.9 tire No.: P-22 P-23 Drift thrust (r.u.): 100 115Performance on a damp circuit comprising bends (r.u.): travelling time:100  97 behaviour mark: 100 110

1. A process for preparing a tread for a tire comprising: forming amixture by incorporating in at least one diene elastomer more than 50phr of an inorganic filler as reinforcing filler, between 2 and 15 phrof an inorganic filler-diene elastomer coupling agent, between 1 and 10phr of a methylene acceptor; and optionally including carbon black,present in an amount less than 20% by weight relative to the quantity oftotal reinforcing filler, the quantity of total reinforcing filler beingthe sum of inorganic filler and carbon black; thermomechanicallykneading the mixture, in one or more stages until a maximum temperatureof between 130° C. and 200° C. is reached; cooling the mixture to atemperature of less than 100° C.; and then incorporating: between 0.5and 5 phr of a methylene donor, and a vulcanization system; kneading themixture having the vulcanization system until a maximum temperature ofless than 120° C.; and then extruding or calendaring a rubbercomposition thus obtained in the form of a sulfur-vulcanizable tiretread, curing the sulfur vulcanizable tire tread; and obtaining a tiretread having a rigidity gradient that increases radially from a surfaceof the tread towards a inside of the tread after mechanical running-inof the tire.
 2. The process of claim 1, wherein the diene elastomer isselected from the group consisting of polybutadienes, syntheticpolyisoprenes, natural rubber, butadiene copolymers, isoprene copolymersand mixtures of these elastomers.
 3. The process according to claim 1,wherein the inorganic filler is a siliceous or aluminous filler.
 4. Theprocess according to claim 3, wherein the inorganic filler is silica. 5.The process according to claim 1, wherein the quantity of inorganicfiller is between 60 and 120 phr.
 6. The process according to claim 1,wherein the inorganic filler-diene elastomer coupling agent is anorganosilane or a polyorganosiloxane.
 7. The process according to claim6, wherein the coupling agent is a polysulphurized alkoxysilane.
 8. Theprocess according to claim 1, wherein the quantity of methylene acceptoris between 2 and 8 phr.
 9. The process according to claim 1, wherein thequantity of methylene donor is between 0.5 and 3.5 phr.
 10. The processaccording to claim 1, wherein the methylene acceptor is at least onephenolic resin.
 11. The process according to claim 10, wherein themethylene acceptor is a novolac phenolic resin.
 12. The processaccording to claim 1, wherein the methylene donor is selected from thegroup consisting of hexamethylenetetramine (HMT),hexamethoxymethylmelamine (H3M), hexaethoxymethylmelamine, polymers ofpara-formaldehyde, N-methylol derivatives of melamine, and mixtures ofthese compounds.
 13. The process according to claim 12, wherein themethylene donor is selected from the group consisting of HMT, H3M, andmixtures of these compounds.
 14. The process according to claim 1,wherein the quantity of methylene acceptor represents between 2% and 15%by weight relative to the weight of inorganic filler.
 15. The processaccording to claim 1, wherein the quantity of methylene donor representsbetween 10% and 30% by weight relative to the weight of the methyleneacceptor.
 16. The process according to claim 2, wherein the dieneelastomer is a butadiene-styrene (SBR) copolymer.
 17. The processaccording to claim 16, wherein the SBR copolymer has a styrene contentof between 20% and 30% by weight, a content of vinyl bonds of thebutadiene fraction of between 15% and 65%, a content of trans-1,4 bondsof between 20% and 75% and a glass transition temperature of between−20° C. and −55° C.
 18. The process according to claim 17, wherein theSBR copolymer is an SBR prepared in solution.
 19. The process accordingto claim 2, wherein the diene mixture is a mixture of SBR copolymer andpolybutadiene.
 20. The process according to claim 19, wherein thepolybutadiene has more than 90% cis-1,4 bonds.
 21. The process accordingto claim 1, wherein the inorganic filer represents more than 80% of thetotal reinforcing filler.
 22. The process according to claim 1, whereinthe inorganic filler is used in a mixture with carbon black.
 23. Theprocess according to claim 22, wherein the carbon black is present in anamount of between 2 and 15 phr.
 24. The process according to claim 23,wherein the carbon black is present in an amount of between 4 and 12phr.
 25. The process according to claim 7, wherein the coupling agent isat least one bis-(C1–C4)alkoxylsilyl(C1–C10)alkyl polysulphide.
 26. Theprocess according to claim 25, wherein the coupling agent is abis-(C1–C4) alkoxylsilylpropyl polysulphide.
 27. The process accordingto claim 26, wherein the coupling agent is a bis 3-triethoxysilylpropyldisulphide or tetrasulphide.
 28. The process of claim 1, wherein thecarbon black is present in an amount of less than 10% by weight relativeto the quantity of total reinforcing filler.
 29. A tread for a tireproduced in accordance with the process of claim
 1. 30. The tread ofclaim 29, wherein the tread is produced in accordance with the processof claim
 8. 31. The tread of claim 29, wherein the tread is produced inaccordance with the process of claim
 9. 32. The tread of claim 29,wherein the tread is produced in accordance with the process of claim14.
 33. The tread of claim 29, wherein the tread is produced inaccordance with the process of claim
 15. 34. The tread of claim 29,wherein the tread is produced in accordance with the process of claim22.
 35. The tread of claim 29, wherein the tread is produced inaccordance with the process of claim
 23. 36. A tire comprising a tiretread, wherein the tire tread is produced in accordance with the processof claim
 1. 37. The tire of claim 29, wherein the tire tread is producedin accordance with the process of claim
 8. 38. The tire of claim 36,wherein the tire tread is produced in accordance with the process ofclaim
 9. 39. The tire of claim 36, wherein the tire tread is produced inaccordance with the process of claim
 14. 40. The tire of claim 36,wherein the tire tread is produced in accordance with the process ofclaim
 15. 41. The tire of claim 36, wherein the tire tread is producedin accordance with the process of claim
 22. 42. The tire of claim 36,wherein the tire tread is produced in accordance with the process ofclaim 23.